1. Field of the Invention
The invention relates to a wireless communication system such as a local area network, and more particularly to a wireless communication including a radio-signal base station with which a mobile wireless terminal such as a portable computer or a cellular phone makes communication with reduced power consumption.
The invention relates further to a method of making wireless communication between a radio-signal base station and a mobile wireless system, and to a mobile wireless terminal used in the above-mentioned wireless communication system.
2. Description of the Related Art
With development of wireless communication technology, a mobile wireless terminal such as a cellular phone, a personal handy-phone system (PHS), a personal digital assistant (PDA) or a handy-sized computer is widely used. Such a mobile wireless terminal is designed to make connection with Internet through an access point without communication cables, and transmit data to and receive data from other data-processing devices through a communication network such as a local area network (LAN), a wide area network (WAN) or a personal area network (PAN).
Recently, wireless LAN communication equipment called a hot spot is equipped in public spaces such as stations or cafes. Hence, a person having a mobile wireless terminal can watch homepages and/or transmit and receive e-mails through Internet by dropping in such hot spots.
A company can reduce costs for re-construction of communication cables by adopting such wireless communication system as mentioned above, even if the company relocates. Furthermore, the above-mentioned wireless communication system provides an advantage that communication with a person who often moves can be readily made without direct connection of his/her terminal with a fixed cable.
In the above-mentioned wireless communication system, since a mobile wireless terminal makes wireless communication with a radio-signal base station acting as an access point, it is necessary for a mobile wireless terminal not to be directly connected to a power-feeding line. That is, a mobile wireless terminal is necessary to be driven by a battery. As exceptional cases, a mobile wireless terminal is connected directly a power-feeding line through an adapter, only when a mobile wireless terminal is not in use. Hence, many attempts have been made to reduce power consumption of a mobile wireless terminal for ensuring long-time use, or for reducing a weight of a battery.
For instance, Japanese Patent Application Publication No. 2002-208887 has suggested a cellular phone which reduces power consumption while it is out of a communication area. If a cellular phone is out of a communication area, a user cannot make communication with other cellular phones through his/her cellular phone, and an available channel is not ensured. Hence, his/her cellular phone carries out channel-scanning for searching an available channel, resulting in much power-consumption of a battery. Hence, the suggested cellular phone is designed to temporarily cut off a power when the cellular phone is out of a communication area for a predetermined period of time, preventing power consumption while the cellular phone cannot make communication with other phones.
Attempts have been made to reduce power consumption by partially varying functions of a circuitry of a mobile wireless terminal under predetermined conditions. For instance, Japanese Patent Application Publication No. 2000-278165 has suggested a method of reducing power consumption in a CDMA (Code Division Multiple Access) type cellular phone. In the suggested method, the number of bits in A/D conversion in a mode of waiting for a call is set smaller than the same while communication is being made, reducing power consumption in A/D conversion. This ensures reduction in power consumption in a mode of waiting for a call.
Japanese Patent Application Publication No. 2002-118874 has suggested a mobile wireless terminal including a radio-signal transceiver which is turned off while the mobile wireless terminal is in a stand-by mode, to thereby reduce power consumption.
FIG. 1 is a flow-chart showing steps carried out in the suggested mobile wireless terminal.
While the mobile wireless terminal is in a stand-by mode, a user actuates a predetermined key for starting on-off control, in step S101.
Then, the mobile wireless terminal checks whether the radio-signal transceiver is in operation or not, in step S102.
If the mobile wireless terminal is in operation (YES in step S102), the mobile wireless terminal checks whether a user carried out a step for stopping operation of the radio-signal transceiver, in step S103.
If a user carried out the step (YES in step S103), the mobile wireless terminal informs a network that operation of the radio-signal transceiver is stopped, in step S104. Then, the mobile wireless terminal stops operation of the radio-signal transceiver, in step S105. Then, the mobile wireless terminal returns to a stand-by mode.
If a user did not carry out the step (NO in step S103), the mobile wireless terminal returns to a stand-by mode without stopping operation of the radio-signal transceiver.
If the mobile wireless terminal is not in operation (NO in step S102), it is checked whether a user carries out steps for operation of the radio-signal transceiver, in step S106.
If a user carries out the steps (YES in step S106), the radio-signal transceiver starts its operation, in step S107. Then, the mobile wireless terminal informs a network that the radio-signal transceiver starts its operation, in step S108. Then, the mobile wireless terminal returns to a stand-by mode.
If a user does not carry out the steps (NO in step S106), the mobile wireless terminal returns to a stand-by mode.
The mobile wireless terminal suggested in Japanese Patent Application Publication No. 2002-118874 is accompanied with problems that a user has to carry out some steps, and a user may wrongly carry out steps, because the radio-signal transceiver is turned on or off manually by a user.
Hence, there has been suggested a mobile wireless terminal which automatically turns on or off a radio-signal transceiver.
For instance, Japanese Patent Application Publication No. 7-131404 has suggested a mobile wireless terminal into which a first time zone in which calls are expected to receive and a second time zone in which calls are not expected to receive, such as night, are stored in advance. In the first time zone, receipt-timing signals are transmitted at a shorter interval for rapidly detecting receipt of signals. In the second time zone, receipt-timing signals are transmitted at a longer interval for reducing power consumption in a battery.
However, the first time zone is generally much longer than the second time zone. Hence, though power consumption in a battery is reduced in the second time zone, power consumption is not reduced at all in the first time zone longer than the second time zone. Totally, power consumption is not so reduced.
Apart from the above-mentioned suggestions, there has been suggested a wireless communication system for the purpose of reduction in power consumption. The wireless communication system is called wireless LAN system, in which mobile wireless terminals are designed to selectively operate in a power-saving mode for reducing power-consumption and a normal mode. When each of the mobile wireless terminals transfers to a power-saving mode from a normal mode, each of the mobile wireless terminals informs a radio-signal base station of such mode-transfer.
FIG. 2 is a time-chart of the above-mentioned wireless communication system. FIG. 2-(A) illustrates signals transmitted from a radio-signal base station, FIG. 2-(B) illustrates signals received in a first mobile wireless terminal which is in a normal mode, FIG. 2-(C) illustrates signals received in a second mobile wireless terminal which is in a power-saving mode, and FIG. 2-(D) illustrates signals received in a third mobile wireless terminal which is in a power-saving mode.
As illustrated in FIG. 2-(A), a radio-signal base station transmits an informative signal 121 or a beacon at a predetermined interval. An informative signal 121A to which a transmission-traffic displaying message is attached is transmitted from a radio-signal base station at an interval defined as M×T wherein T indicates an interval at which the informative signals 121 are transmitted, and M indicates a positive integer equal to or greater than two (2). Immediately after transmission of the informative signal 121A, a multiple-address packet (multi-cast or broadcast) 122 is transmitted from a radio-signal base station. Hereinbelow, an interval at which the informative signals 121A are transmitted is called “DTIM interval”.
In FIG. 2, DTIM interval is illustrated as 3T as an example. The multiple-address packet informs the first to third mobile wireless terminals of information for settling physical addresses of them.
Each of the first to third mobile wireless terminals is designed to be able to select a normal mode in which power is not saved and a power-saving mode in which power consumption is reduced. As illustrated in FIG. 2-(B), the first mobile wireless terminal which is in a normal mode is in active condition 123 in which the first mobile wireless terminal can receive packets from a radio-signal base station in all of time zones. In contrast, as illustrated in FIG. 2-(C), the second mobile wireless terminal which is in a power-saving mode detects the interval T included in the informative signal transmitted from a radio-signal base station, and puts a circuit for making communication into active condition 124 at the interval T in synchronization with the informative signals 121.
The third mobile wireless terminal is designed to reduce power consumption to a greater degree than the second mobile wireless terminal. As illustrated in FIG. 2-(D), the third mobile wireless terminal is periodically put into active condition 125 in which the third mobile wireless terminal can receive packets from a radio-signal base station, at time zones for receiving the informative signals 121A and the subsequently transmitted multiple-address packets 122.
By lengthening an interval at which the third mobile wireless terminal is put into the active condition 125, it would be possible to lengthen a pause time in which power consumption can be reduced.
For instance, it is assumed that no packet signals 128 are transmitted to a certain mobile wireless terminal at a certain time zone. In such a time zone, even if the mobile wireless terminal is put into the active condition 124 at an interval at which the informative signals 121 are received, the mobile wireless terminal is never informed of receipt of the packet signals 128 by the informative signals 121. This results in repetition in receipt of the informative signals 121 in vain. Hence, in a time zone in which no traffic exists, the third mobile wireless terminal is designed to receive the informative signals 121 only at a minimum time zone in which the multiple-address packets 122 are transmitted to all of the mobile wireless terminals in multiple-address communication.
Hereinbelow is explained how packets are received with reference to the first mobile wireless terminal which is in a normal mode (see FIG. 2-(B)) and the second mobile wireless terminal which is in a power-saving mode (see FIG. 2-(C)).
It is now assumed that packets signals 128 addressed to a certain mobile wireless terminal are transmitted to a radio-signal base station at time t1 from a server (not illustrated) connected to LAN.
A radio-base signal station checks whether the mobile wireless terminal to which the packet signals 128 are addressed is in a normal mode or in a power-saving mode. If the mobile wireless terminal is in a normal mode (see FIG. 2-(B)), the radio-signal base station starts transmitting the packet signals 126 to the mobile wireless terminal at time t1.
In contrast, if the packet signals 128 are addressed to the second mobile wireless terminal (see FIG. 2-(C)), since the mobile wireless terminal is in a power-saving mode, the packet signals 128 cannot be received by the mobile wireless terminal, even if a radio-signal base station transmits the packet signals 128 at time t1 to the second mobile wireless terminal. Hence, a radio-signal base station stores the packet signals 128 into a memory equipped in the radio-signal base station. Then, a radio-signal base station combines information indicating that the packet signals 128 addressed to the second mobile wireless terminal are temporarily stored, to the informative signal 121 to be transmitted at time t2, and transmits the informative signal 121 to the second mobile wireless terminal, as shown with an arrow 131.
At time t2, the second mobile wireless terminal is in the active condition 124. Hence, the second mobile wireless terminal can receive the informative signal 121, and becomes aware that the packet signals 128 addressed to the second mobile wireless terminal are stored in a radio-signal base station, by analyzing the received informative signal 121.
Then, the second mobile wireless terminal transmits a request 132 to a radio-signal base station to transmit the packet signals 128 to the second mobile wireless terminal. On receipt of the request 132, a radio-signal base station transmits the packet signals 128 as data packets 133 to the second mobile wireless terminal at time t3.
The second mobile wireless terminal is in a condition for waiting for the packet signals 128 to be transmitted from a radio-signal base station. Hence, though not illustrated, the second mobile wireless terminal is kept in the active condition for receiving the packet signals 128, after transmission of the request 132.
As explained above, comparing the first mobile wireless terminal being in a normal mode to the second mobile wireless terminal being in a power-saving mode, there is time delay in a time at which data packet starts being transmitted. Specifically, such time delay is equal to a time difference between time t1 and time t3. Accordingly, the conventional wireless communication system illustrated in FIG. 2 is accompanied with problems that when real-time communication packets such as voices and images are transmitted to a radio-signal base station, time delay is caused, and a break is caused in packet transmission, resulting in that reproducibility of data is degraded.
The third mobile wireless terminal (FIG. 2-(D)) pauses in a longer time than the second mobile wireless terminal, though it is dependent on a timing at which the packet signals addressed to the third mobile wireless terminal arrive at a radio-signal base station. Hence, the third mobile wireless terminal may receive the informative signal 121 indicating that the packet signals addressed to the third mobile wireless terminal are stored in a radio-signal base station, later than the second mobile wireless terminal. This results in an increase in the above-mentioned time delay in comparison with the second mobile wireless terminal. Accordingly, reproducibility of data to be reproduced at real-time is further degraded.
A radio-signal base station has to store therein packet signals addressed to the second and third mobile wireless terminals, by a period of time equal to the above-mentioned time delay. Hence, as the time delay becomes longer, a radio-signal base station has to have a memory having a greater capacity for storing packet signals therein.
Japanese Patent Application Publication No. 2001-186567 has suggested a mobile communication system in which a control station detects calling traffic in each of calling sections, and determines the variable number K of repetition of transmission of a calling signal in accordance with the detected calling traffic. The control station informs mobile stations of the number K, and transmits calling signals K times over K frames. The mobile stations intermittently receive the calling signals every K frames.
Japanese Patent Application Publication No. 11-136741 has suggested a wireless data-communication system in which a master device is informed that a radio-signal base station controlling a wireless slave station receives data on behalf of the wireless slave station.
Japanese Patent Application Publication No. 2002-164835 has suggested a mobile terminal including a selector for turning a stand-by mode on or off, a processor for dealing with actions other than transmission and receipt of signals when the stand-by mode is turned off through the selector, and a second processor for turning the stand-by mode on when a user is going to transmit signals, and turning the stand-by mode off after signal transmission has been carried out.
Japanese Patent Application Publication No. 9-275587 has suggested a mobile wireless terminal including a timer. While the timer is in operation, power is not supplied to a receiver, and after the timer stops its operation, power is supplied to the receiver. Thus, the mobile wireless terminal intermittently receives data out of a service area. A time at which the timer starts or stops its operation can be determined by a user.
Japanese Patent Application Publication No. 8-172387 has suggested a mobile wireless terminal which is capable of varying a timing at which signals transmitted from a base station are received such that a signal receiver of the mobile wireless terminal is kept on for a long time when the mobile wireless terminal is in a service area, and for a short time when the mobile wireless terminal is out of a service area.
Japanese Patent Application Publication No. 63-296419 has suggested a wireless communication system in which a base station periodically transmits an informative signal to a mobile wireless terminal. While the mobile wireless terminal is in a stand-by mode in which the mobile wireless terminal intermittently receives signals from the base station at an interval, if the mobile wireless terminal does not receive signals other than the informative signals for a certain period of time, the mobile wireless terminal lengthens the interval.
Japanese Patent Application Publication No. 6-140984 has suggested a mobile wireless terminal which shortens a period of time in which a battery-saving mode is off, when the mobile wireless terminal does not receive signals from a base station, and turns a battery-saving mode on while the mobile wireless terminal is receiving signals from a base station.